The use of opiates to treat pain is a significant cause of addiction, but there are no adequate alternatives. Neurokinins and opioids are two neuropeptides important in acute and chronic pain; hence mechanisms that control their release in the spinal cord could be targets for the development of new analgesics. Neurokinin release from primary afferent terminals appears to be inhibited by muopioid, cannabinoid, alpha2 adrenergic and GABAB receptors, but their mechanisms of action, relative efficacy and physiological relevance have not been established. Receptors that modulate spinal opioid release are virtually unknown. Long term objectives: A) to create a model integrating the actions of receptors that control neurokinin release from primary afferents; B) to understand how different physiological states determine the release of opioids in the spinal cord, identifying the neural pathways and neurotransmitter systems involved. Specific Aims: 1) to determine whether opioid, cannabinoid, adrenergic and GABAB receptors inhibit neurokinin release from primary afferent terminals using a common signalling pathway; 2) to identify neurotransmitter systems that inhibit opioid release in the dorsal horn; 3) to identify neuronal pathways that evoke opioid release in the dorsal horn. Research Design: Rat spinal cord slices will be electrically stimulated at the dorsal root to evoke neurokinin release, or at the dorsal horn or the dorsolateral funiculus to evoke opioid release. Internalization of neurokinin 1 receptors and mu-opioid receptors will be used to measure neurokinin and opioid release, respectively. This will be validated using immunoassays and transgenic mice. In vivo experiments will compare the effect of drugs on behavioral pain responses and receptor internalization in the dorsal horn. They will also determine whether noxious stimuli or stimulation of the periaqueductal gray or the nucleus raphe magnus produce mu-opioid receptor internalization in the spinal cord. The neural pathways involved will be studied using selective spinal cord lesions.